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• Most optimal way to detect if black (or any color pixels) exist in an image file?

  - by Zando

  What's the best and most flexible algorithm to detect any black (or colored pixel) in a given image file? Say I'm given an image file that could, say, have a blue background. And any non blue pixel, including a white pixel, is counted as a "mark". The function returns true if there are X number of pixels that deviate from each other at a certain threshold. I thought it'd be fastest to just simply iterate through every pixel and see if its color matches the last. But if it's the case that pixel (0,0) is deviant, and every other pixel is the same color (and I want to allow at least a couple deviated pixels before considering an image to be "marked), this won't work or be terribly efficient.

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• Image analysis on the iPhone

  - by user362808

  Hi, guys! Hopefully a quick one. Working on a devious little algorithm that'll automatically scan a user's iPhone for a photo of some emotional poignance (e.g. go back in time a ways, look for one of a group of photos that are proximally timestamped, etc.). Need a quick-and-dirty way of picking out a photo containing two people in close proximity to each other. I know that the OpenCV Python lib [for image processing] works on the iPhone, I'm just curious whether it's actually the best way of doing this (from scratch or otherwise). Cheers!

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• how to get the value of checksum in php header

  - by sumit

  I have a header in php which contains a link like <?php header("Location: "."https://abc.com/ppp/purchase.do?version=3.0&". "merchant_id=<23255>&merchant_site_id<21312>&total_amount=<69.99>&". "currency=<USD>&item_name_1=<incidentsupporttier1>&item_amount_1=<1>&". "time_stamp=<2010-06-14.14:34:33>&**checksum=<calculated_checksum>**"); ?> when i run this page the value of checksum is calculated and the link is opened now how checksum is calculated? calculated_checksum=md5(abc); md5 is an algorithm which calculates the value of checksum based on certain values inside the bracket. now i want to know how can i pass the value of checksum in the header url

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• Will a source-removal sort always return a maximal cycle?

  - by Jason Baker

  I wrote a source-removal algorithm to sort some dependencies between tables in our database, and it turns out we have a cycle. For simplicity, let's say we have tables A, B, C, and D. The edges are like this: (A, B) (B, A) (B, C) (C, D) (D, A) As you can see, there are two cycles here. One is between A and B and another is between all four of them. Will this type of sort always choke on the largest cycle? Or is that not necessarily the case?

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• Best .Net/C# book for an interview

  - by DrunkenBeard

  Hi, I'm having an interview for an internship in the coming days and would like to know if there are any books that are particulary suited for preparing interviews. The interview is not particulary hard, just some questions about .Net and C# plus an algorithm to code. I tried C# in a Nutshell but it goes in too much detail for what I want to achieve. I'm more looking for a good yet small book that explains the important concepts of the .Net framework and C#, I already know the details, just want formal explanations so I'm confident when answering. Thanks :)

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• 16 millisecond quantization when sending/receivingtcp packets

  - by MKZ

  Hi, I have a C++ application running on windows xp 32 system sending and receiving short tcp/ip packets. Measuring (accurately) the arrival time I see a quantization of the arrival time to 16 millisecond time units. (Meaning all packets arriving are at (16 )xN milliseconds separated from each other) To avoid packet aggregation I tried to disable the NAGLE algorithm by setting the IPPROTO_TCP option to TCP_NODELAY in the socket variables but it did not help I suspect that the problem is related to the windows schedular which also have a 16 millisecond clock.. Any idea of a solution to this problem ? Thanks

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• DLL Load Failed: %1 is not a valid win32 application

  - by glittershark

  So I have a situation where I need to make binary patches and then be able to apply them from within python. I found bsdiff which looks like a great algorithm and has a python extension module, but that extension module doesn't have a windows installer past Python 2.5 Alright, so having never even written C seriously, let alone attempted a python extension module, I set out to compile it myself. After a few hours of bashing my head against a DLL load error with MinGW32, I managed to get it compiled and built using the setup.py provided in the source of the project. However, upon importing the resulting bsdiff module, I get: ImportError: DLL load failed: %1 is not a valid Win32 application. Any advice?

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• Error occurs while using SPADE method in R

  - by Yuwon Lee

  I'm currently mining sequence patterns using SPADE algorithm in R. SPADE is included in "arulesSequence" package of R. I'm running R on my CentOS 6.3 64bit. For an exercise, I've tried an example presented in http://en.wikibooks.org/wiki/Data_Mining_Algorithms_In_R/Sequence_Mining/SPADE When I tried to do "cspade(x, parameter = list(support = 0.4), control = list(verbose = TRUE))" R says: parameter specification: support : 0.4 maxsize : 10 maxlen : 10 algorithmic control: bfstype : FALSE verbose : TRUE summary : FALSE preprocessing ... 1 partition(s), 0 MB [0.096s] mining transactions ... 0 MB [0.066s] reading sequences ...Error in asMethod(object) : 's' is not an integer vector When I try to run SPADE on my Window 7 32bit, it runs well without any error. Does anybody know why such errors occur?

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• Best way to search a point across several polygons

  - by user1474341

  I have a requirement whereby I need to match a given point (lat,lon) against several polygons to decide if there is a match. The easiest way would be to iterative over each polygon and apply the point-in-polygon check algorithm, but that is prohibitively expensive. The next optimization that I did was to define a bounding rectangle for each polygon (upper bound, lower bound) and iteratively check the point against the bounding box (fewer comparisons as against checking all the points in the polygon). Is there any other optimization possible? Would a spatial index on the bound rectangle points or a geohash help ? Any guidance would be greatly appreciated. Thanks!

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• can a program written in C be faster than one written in OCaml and translated to C?

  - by Ole Jak

  So I have some cool Image Processing algorithm. I have written it in OCaml. It performs well. I now I can compile it as C code with such command ocamlc -output-obj -o foo.c foo.ml (I have a situation where I am not alowed to use OCaml compiler to bild my programm for my arcetecture, I can use only specialy modified gcc. so I will compile that programm with sometyhing like gcc -L/usr/lib/ocaml foo.c -lcamlrun -lm -lncurses and Itll run on my archetecture.) I want to know in general case can a program written in C be faster than one written in OCaml and translated to C?

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• C# remove duplicates from List<List<int>>

  - by marseilles84

  I'm having trouble coming up with the most efficient algorithm to remove duplicates from List<List<int>>, for example (I know this looks like a list of int[], but just doing it that way for visual purposes: my_list[0]= {1, 2, 3}; my_list[1]= {1, 2, 3}; my_list[2]= {9, 10, 11}; my_list[3]= {1, 2, 3}; So the output would just be new_list[0]= {1, 2, 3}; new_list[1]= {9, 10, 11}; Let me know if you have any ideas. I would really appreciate it.

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• How to stop a MFC/win32 control re-painting

  - by John

  I have a CRichEditCtrl, and an algorithm works on its contents in-place, iterating through the contents and making changes. This seems to cause it to re-paaint every time, which causes flickering and a noticeable time to complete the operation, especially if the control starts scrolling. I wondered if there's a neat way for me to stop it repainting for a period. It's actually a custom subclass of CRichEditCtrl in use so I was thinking something like: void MyCustomEditCtrl::DoAlgorithm() { enableRepaint(false); ... //to the actual work ... enableRepaint(true); } What's the neatest approach? A custom on-paint handler which does nothing when an operation is in progress? Or can repainting actually be disabled?

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• Modify an MP3 slightly to change the data appearence

  - by Léon Pelletier

  I'm thinking about encrypting MP3s in a database, so that when user is downloading them with his software desktop player, only the software can decrypt them. This part is not a problem. The problem is I don't want a user to upload an mp3 to the database, then check which changes have been made to the file so he can reverse-engineer the file or at least see which algorithm is used to encrypt the files. So, user uploads MP3-A, then it becomes MP3-B because it has been modified, and I encrypt it to MP3-C. And when decrypted, it sounds 99.99% like MP3-A. I know MP3 format is lossy, but I wonder if there's a way to convert audio with limited loss, or if I need to forget it right now.

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• Optimal diff between object lists in Java

  - by Philipp

  I have a List of Java objects on my server which is sent to the client through some serialization mechanism. Once in a while the List of objects gets updated on the server, that is, some objects get added, some get deleted and others just change their place in the List. I want to update the List on the client side as well, but send the least possible data. Especially, I don't want to resend Objects which are already available on the client. Is there a library available which will produce some sort of diff from the two lists, so that I can only send the difference and the new Objects accross the wire? I have found several Java implementation of the unix diff command, but this algorithm is unpractical for order changes. ie. [A,B,C] - [C,B,A] could be sent as only place changes [1-3] [3-1], while diff will want to resend the whole A and C objects (as far as I understand).

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• deleting and reusing a temp table in a stored precedures

  - by Sheagorath

  Hi I need to SELECT INTO a temp table multiple times with a loop but I just can't do it, because after the table created( in SELECT into you can't simply drop the table at the end of the loop because you can't delete a table and create it again in the same batch. so how can I delete a table in a stored procedure and create it again? here is a snippet of where I am actualy using the temp table which is supposed to be a pivoting algorithm: WHILE @offset<@NumDays BEGIN SELECT bg.*, j.ID, j.time, j.Status INTO #TEMP1 FROM #TEMP2 AS bg left outer join PersonSchedule j on bg.PersonID = j.PersonID and bg.TimeSlotDateTime = j.TimeSlotDateTime and j.TimeSlotDateTime = @StartDate + @offset DROP TABLE #TEMP2; SELECT * INTO #TEMP2 FROM #TEMP1 DROP TABLE #TEMP1 SET @offset = @offset + 1 END

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• how to specify a pointer to an overloaded function?

  - by davka

  I want to pass an overloaded function to the std::for_each() algorithm. e.g.: void f(char c); void f(int i); std::string s("example"); std::for_each(s.begin(), s.end(), f); I'd expect the compiler to resolve f() by the iterator type. Apparently, it (gcc 4.1.2) doesn't do it. So, how can I specify which f() I want? thanks a lot

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• Server Side code Pushing Data to client Browser while current thread is busy Comet (programming)

  - by h_power11

  Hello Friends, I am writing one simple web page with bunch of textboxes and a button control. Now when user finished editing the values on this text boxes user has to click the button and this button invoke heavily process intensive algorithm on server side code based on the data received from client (Textboxes) And it could some time takes up to 30 to 45 minutes to complete the whole operation so the current thread is still inside the button click event handler function. That background task only provides one event, and the web page subscribes to it to get some text data after each stage of processing I was wandering if there is any way I can keep user up-to-date with what is the current progress on that background task. I have div element to print the current status information So I am looking for some sort of reverse mechanism then "get" and "post". I have read some articles on the Comet (programming) but I can't find any easy or definitive answer Thanks in advance

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• How to stable_sort without copying?

  - by Mehrdad

  Why does stable_sort need a copy constructor? (swap should suffice, right?) Or rather, how do I stable_sort a range without copying any elements? #include <algorithm> class Person { Person(Person const &); // Disable copying public: Person() : age(0) { } int age; void swap(Person &other) { using std::swap; swap(this->age, other.age); } friend void swap(Person &a, Person &b) { a.swap(b); } bool operator <(Person const &other) const { return this->age < other.age; } }; int main() { static size_t const n = 10; Person people[n]; std::stable_sort(people, people + n); }

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• log (1-var) operation in C

  - by heike

  I am trying to code this algorithm. I am stuck in the part of log((1.0-u)/u))/beta; As I understand, I can not get the result of this in C, as it will always return me with negative value log (returning imaginary value). Tried to print the result of log(1-5) for instance, it gives me with Nan. How can I get the result of double x = (alpha - log((1.0-u)/u))/beta then? Would appreciate for any pointers to solve this problem. Thank you

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• strategy to allocate/free lots of small objects

  - by aaa

  hello I am toying with certain caching algorithm, which is challenging somewhat. Basically, it needs to allocate lots of small objects (double arrays, < 256 elements), with objects accessible through mapped value, map[key] = array. time to initialized array may be quite large, generally more than 10 thousand cpu cycles. By lots I mean around gigabyte in total. objects may need to be popped/pushed as needed, generally in random places, one object at a time. lifetime of an object is generally long, minutes or more, however, object may be subject to allocation/deallocation several times during duration of program. What would be good strategy to avoid memory fragmentation, while still maintaining reasonable allocate deallocate speed? I am using C++, so I can use new and malloc. Thanks. I know there a similar questions on website, http://stackoverflow.com/questions/2156745/efficiently-allocating-many-short-lived-small-objects, are somewhat different, thread safety is not immediate issue for me.

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• database encryption questions

  - by 5YrsLaterDBA

  We are using Sybase SQL Anywhere 11. We need to encrypt some of our tables in our database. I followed the instruction and did it. We selected the "strong" option with encryptionKey and AES256_FIPS algorithm. But there are something I am not clear about them. It will require encryptonKey when we create the database, remove the database and start the database server but it will NOT require encryptionKey when we stop the database server and connect to the server to create tables and add data. Why there is NO encryptionKey asked when we connect to it or try to stop the server? I am doing something wrong? don't know how to test the encryption? I still can see all plain text in the encrypted tables when I use Sybase Central tool. If somebody knows the database user name and password, he/she can connect to the database and read the content without the encryptionKey. is this right?

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• Dividing n-bit binary integers

  - by Julian

  Was wondering if anyone could help me with creating a pseudocode for how to go about dividing n-bit binary integers. Here is what I'm thinking could possibly work right now, could someone correct this if I'm wrong: divide (x,y) if x=0: return (0,0) //(quotient, remainder) (q,r) = divide(floor(x/2), y) q=2q, r=2r if x is odd: r = r+1 if r >= y: r = r-y, q = q+1 return (q,r) Would you guys say that this general pseudocode algorithm would accomplish the intended task of dividing n-bit numbers or am I missing something in my psuedocode before I start coding up something that's wrong?

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• What machine learning algorithms can be used in this scenario?

  - by ExceptionHandler

  My data consists of objects as follows. Obj1 - Color - shape - size - price - ranking So I want to be able to predict what combination of color/shape/size/price is a good combination to get high ranking. Or even a combination could work like for eg: in order to get good ranking, the alg predicts best performance for this color and this shape. Something like that. What are the advisable algorithms for such a prediction? Also may be if you can briefly explain how I can approach towards the model building I would really appreciate it. Say for eg: my data looks like Blue pentagon small $50.00 #5 Red Squre large $30.00 #3 So what is a useful prediction model that I should look at? What algorithm should I try to predict like say highest weightage is for price followed by color and then size. What if I wanted to predict in combinations like a Red small shape is less likely to higher rank compared to pink small shape . (In essence trying to combine more than one nominal values column to make the prediction)

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• Updateable Priority Queue

  - by user1427661

  Is there anything built into the C++ Standard Library that allows me to work in a priority queue/heap like data structure (i.e., can always pop the highest value from the list, can define how the highest value is determined for custom classes, etc.) but allows me to update the keys in the heap? I'm dealing with fairly simple data, pairs to be exact, but I need to be able to update the value of a given key within the heap easily for my algorithm to function. WHat is the best way to achieve this in C++?

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• "Refreshing" an XML feed on iPhone/Mac OSX

  - by Steve

  Hi all, I'm curious for those of you who are building iPhone apps based on REST/SOAP/XML-RPC or simply pulling down a dynamic XML feed, what does it mean exactly to you when a user says 'refresh' the feed? The straight forward way is to populate some collection, say an NSMutableArray, with whatever you bring down from the feed. If a widget on the UI is available to refresh, I typically do something like: [myMutableArray removeAllObjects]; // follow steps to repopulate myMutableArray It seems this is the least efficient algorithm for refreshing an XML feed. For instance many folks who are building Twitter clients, are appending changes to their existing feed, versus bringing down the entire feed in its complete form again. What kind of algorithms are you using to "refresh" your models when speaking to a server-side data source? Thanks all.

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